The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs.
For example, in deep sub-micron integrated circuit technology, a static random access memory (SRAM) cell has become a popular storage unit of high speed communication, image processing and system-on-chip (SOC) products. The amount of SRAMs in microprocessors and SOCs increases to meet the performance requirement in each new technology generation. Although existing SRAM cells have been generally adequate for their intended purposes, they have not been entirely satisfactory in every aspect.